The coefficients \(B_{n}(T)\)  are called the virial coefficients and they are unique real functions of the temperature. The second virial coefficient \(B_2(T)\)  represents the most significant deviation from the ideal behavior, since it is the prefactor in the term of order \(\rho ^{2}\)  in the series. It is a customary result from equilibrium statistical mechanics (see [1]}) that \(B_2(T)\)  is a radial integral of the pair-potential \(v(r)\)  given by
\(B_2(T)=2\pi \int ^{\infty }_0\big (1-{\rm e}^{-\frac{v(r)}{kT}}\big )r^2{\rm d}r. \) 
